1. Field of the Invention
The present invention relates to a probe for checking linear dimensions of mechanical pieces with a support; an arm movable with respect to the support and including a first element, a second element and a third intermediate element connected in a removable way to the first and second elements; a feeler fixed to the first element of the movable arm for contacting a piece to be checked; and detection means adapted for providing a signal responsive to the position of the movable arm; the intermediate element including a section adapted to break while protecting the probe from accidental impacts against the feeler and the first element of the movable arm.
2. Description of the Prior Art
There already are known co-ordinate measuring machines, grinding machines, machining centers, numerically controlled lathes and bench gauges, measuring heads equipped with position transducers and "touch-trigger" type heads, including movable arms provided with safety devices adapted for preventing the occurrence of any irreparable or serious damages to the head, as a result of accidental impacts striking the feeler or the more exposed part of the movable arm. A first type of safety device is disclosed in the sensing head described in British Pat. No. 1 271 841; in this head the movable stylus rod includes two parts connected by a resilient detachable coupling including a disk and balls biased by leaf springs. A sufficiently violent blow, striking along a direction that is perpendicular to the disk axis, causes the detachment of the portion of the stylus rod that carries the disk and the lower edge of the stylus rod. When this occurs, the portion can be engaged again in the operating position. This solution is subject to the inconvenience that a sufficiently accurate, stable and repeatable positioning of the detachable portion of the movable stylus cannot be guaranteed. Moreover, this resilient coupling safety device is not efficacious in the event the lower edge of the stylus is struck along an axial direction of the stylus.
Another type of safety device, employed in the measuring heads disclosed in U.S. Pat. Nos. 3,122,838 and 4,238,886, foresees frictional devices located between two parts of the movable arm. These devices, too, are subject to various inconveniences: they are not efficacious in preventing damage from occurring as a consequence of blows in all the possible striking directions and they can give rise to not easily perceivable zero-setting drifts that may pass unnoticed to the operator, who, consequently, does not take any action and so relies on inexact measurements detected by the measuring head.
A third type of safety device is used in the measuring head described in U.S. Pat. No. 3,250,012. The movable arm or spindle or this head has a reduced neck portion to assure that the spindle will snap as a consequence of at least those impacts striking along transversal directions with respect to the axis of the spindle. The drawbacks of this solution consist in the necessity of replacing, in case of breakage of the neck portion, the whole spindle and in the absence of protection against impacts along the axial direction of the spindle.
A fourth known type of safety device, employed in the gauging head disclosed in U.S. Pat. No. 3,520,063, comprises a sacrificial stud with two ends threaded into two shafts of the movable arm and with an intermediate section where possible breakages occur. This safety device provides advantages like a considerably rigid but sufficiently fragile coupling between the two shafts of the movable arm and allows, in the event of a stud breakage, the resetting of the head efficiency just by replacing the stud. On the other hand the safety stud does not protect against axial loads (directed along the longitudinal axis of the movable arm and of the stud).